1. Field of the invention
This invention relates to a light emitting diode which can attain stable emission of light with a short wavelength from blue to ultraviolet.
2. Description of the prior art
In recent years, light emitting diodes have been widely used as a display element in various display devices because of their small size, low consumption of power, and stable emission of light with high brightness. Moreover, they have also been put to practical use as a light source for readout of the recorded data in various data processing apparatus. However, light emitting diodes which have been put to practical use so far are those capable of emitting light with a long wavelength from red to green, whereas diodes capable of emitting light with a short wavelength from blue to ultraviolet have not yet been realized.
The emission wavelength of a light emitting diode depends upon the kind of semiconductor materials to be used therein. The semiconductor materials for use in blue light emitting diodes are limited to silicon carbide (SIC, one of the IV--IV group compound semiconductors; Eg (forbidden band width)=3.0 eV for .alpha.-SiC), gallium nitride (GAN, one of the III-V group compound semiconductors; Eg=3.4 eV), zinc sulfide (ZnS, one of the II-VI compound semiconductors; Eg=3.7 eV), and zinc selenide (ZnSe, one of the II-VI group compound semiconductors; Eg=2.7 eV). Examples of the semiconductor materials for use in ultraviolet light emitting diodes include aluminum nitride (AlN, one of the III-V group compound semiconductors; Eg=6.0 eV) and aluminum gallium nitride (Ga.sub.x Al.sub.1-x N, 0&lt;x&lt;1, one of the III-V group compound semiconductors; Eg=3.4 to 6.0 eV).
It is preferred that light emitting diodes are of p-n junction type. This is because electrons and positive holes can be injected into the emission area with high efficiency. However, it is difficult to prepare p-type crystals from the above-mentioned semiconductor materials other than SiC. Even if obtained, such p-type crystals are highly resistive or extremely unstable, so that p-n junction light emitting diodes cannot be produced using the p-type crystals of these semiconductor materials.
In contrast, both p-type and n-type crystals of SiC can readily be obtained, therefore, it is possible to produce light emitting diodes of p-n junction type. See, for example, M. Ikeda et al., J. Appl. Phys., Vol. 50, No. 12, p. 8215 (1979), and L. Hoffmann et al., J. Appl. Phys., Vol. 53, No. 10, p. 6962 (1982). FIG. 5 shows a conventional silicon carbide light emitting diode of the p-n junction type, which comprises a p-n junction formed of a p-SiC substrate 1 and an n-SiC layer 2, an ohmic electrode 6 for the p-SiC and an ohmic electrode 7 for the n-SiC being disposed on the back face of the p-SiC substrate 1 and the upper face of the n-SiC layer 2, respectively. However, the above-mentioned compound semiconductors other than SiC are suitable for light emission with high efficiency because of their direct band structure, whereas SiC has the drawback that even if light emitting diodes of p-n junction type are produced with the use of SiC, their luminous efficiency is low because SiC is one of the indirect band gap materials, therefore, such silicon carbide light emitting diodes cannot be used for practical applications.